Paved surfaces deteriorate after a period of time from a variety of causes which may include the freezing and thawing of moisture which collects in cracks, impacts and wear from vehicles, ground settlement and thermal expansion and contraction. Complete repaving of an extensive deteriorated surface is a costly operation and may require large quantities of petroleum based asphalt or other materials which have recently become scarce. Consequently, the maintenance of roadways and other paved surfaces is increasingly becoming a matter of repairing localized small deteriorated areas where that is possible. This may variously involve filling cracks, potholes or other declivities with new paving material or patching compounds, the limited repaving of separate localized small areas that exhibit such defects, or the addition of a relatively thin overlayer of paving to such areas.
Many localized pavement repair operations of these or other kinds involve heating of the pavement or the surface to be paved or are at least more effective if the area being treated is heated to a depth of several centimeters or more. In repairing asphaltic concrete, for example, better bonding of patch material or new or recycled pavement material to adjacent portions of the pre-existing pavement may be achieved if the old pavement is heated in depth during such repairs. In the case of Portland cement concrete, in-depth heating of the pavement adjacent to a crack or other declivity to be patched with materials such as polymerizable patching compounds provides a better bond and may also promote more rapid and uniform curing of the patch material.
Conventional techniques for heating a paved surface are subject to serious problems. Combustion heaters, infrared heaters or the like apply direct heat only to the surface of the pavement. It is then necessary to rely on the slow process of downward heat conduction to raise the temperature of subjacent regions of the pavement. A severe downward temperature gradient is produced unless the heating period is undesirably prolonged to allow for a gradual distribution of heat by conduction. In order to raise the temperature of the deeper regions of the pavement to a desired extent, it may be necessary to heat the surface region more intensely than is desirable. The overheating of the surface may itself cause deterioration. Ignition of asphalt and related problems, such as smoke pollution, may also be encountered. Overheating of the top portion of the pavement in order to adequately heat the deeper portions is also undesirable in that it is an inefficient use of scarce energy resources.
Use of these conventional heating techniques in conjunction with the localized repair of deteriorated pavement usually results in poor bonding of added patch material or remixed existing pavement material to the adjacent portions of the old pavement. As a practical matter, most existing pavement patches in cracks, potholes or the like tend to be very poorly bonded to the adjacent old pavement and the end result is rapid deterioration of the pavement.
It has recently been recognized in the art that the problems discussed above may be greatly reduced or eliminated by using microwave energy to heat localized areas of pavement in depth in conjunction with repair operations. Microwave energy, which is not itself heat energy, penetrates virtually instantaneously into pavement to depths of several centimeters or more and converts to heat throughout the penetrated volume of pavement by electrical interaction with dielectric constituents of the pavement, most notably by interaction with the rock aggregate content. The result is a rapid and relatively uniform heating of the pavement to substantial depths. An area of asphaltic concrete exhibiting one or more cracks, potholes or the like may be rapidly decomposed into a semi-liquid condition and the constituents, together with new materials which may be added in, may then be remixed and recompacted. Similarly, by heating Portland cement concrete pavements in depth with microwave energy, the curing of polymer patching compounds and the like may be facilitated and improved bonding of the patch material to the concrete is achieved.
As heretofore practiced, pavement repair using microwave heating has been subject to a serious inefficiency from the standpoint of energy use which, in addition to being undesirable in itself, also adversely affects costs. The energy which ultimately appears as heat within the pavement is initially produced by consuming some form of fuel in motor generator means which supply electrical energy to energize the microwave sources. While the motors which drive the generators may variously be diesel engines, gasoline engines, turbines or the like, it is a characteristic of such engines in general that typically around 70% of the energy content of the consumed fuel is discharged as exhaust gas heat or is otherwise dissipated in an unproductive manner.
Disadvantages of known methods for heating pavements in place with microwave energy are not limited to an energy inefficiency. Microwave heating of pavement by known techniques also produces a temperature gradient in the pavement but it is a reversed gradient relative to the gradient produced by conventional pavement heating techniques. In particular, microwave irradiation tends to heat a deeper region of the pavement more strongly than it heats the surface region. The inverted temperature gradient is believed to arise in part from evaporation of moisture which is driven to the pavement surface by microwave heating and other modes of cooling such as heat transfer to ambient air are also more pronounced at the pavement surface, particularly during cold weather.
Although the temperature gradient produced by microwave heating tends to be less severe than the opposite temperature gradient produced by more conventional forms of pavement heating, a still more uniform heating effect is desirable. In addition, some forms of pavement such as old asphaltic concrete tend to have a hard dry crust of the order of several millimeters thick which is believed to be caused by the evaporation over a period of time of the more volatile components of the asphalt binder. Where this condition is encountered in pavement to be repaired, somewhat stronger heating of the surface, relative to the deeper regions of the pavement, would be desirable in order to decompose the crust.
My copending U.S. application Ser. No. 756,365, filed Jan. 3, 1977, and entitled Microwave Method and Apparatus for Reprocessing Pavements, discloses methods and apparatus which reduce or eliminate the problems discussed above. While apparatus disclosed in copending application Ser. No. 756,365, may be used for the repair of specific small localized areas of deteriorated pavement, most of the disclosed systems are primarily designed for large scale reprocessing of lengthy strips of pavement including performing the heating and other operations while traveling continuously along a roadway or the like. A need also exists for less complex and costly apparatus and procedures specifically designed for stationary patching, repaving or other repair operations at relatively small separated deteriorated areas of an expanse of pavement.